This invention pertains generally to continuous wave radar (CW radar) and particularly to radar of such type wherein ranging information may be derived.
It is well known in the art to modulate the transmitted signal from a CW radar so that the range of any detected target may be determined. Thus, it is common practice to modulate a continuous wave carrier signal with a frequency-modulated, or phase-modulated, wave and then, by correlating the echo signal received at any instant in time with a portion of the transmitted signal then being transmitted, producing a signal whose frequency, or phase, is a function of the range of the target.
It is also well known that continuous wave radars suffer from spillover, i.e. direct transmission of energy from the transmitting antenna to the receiving antenna, and from large clutter returns due to reflection from nearby objects. When the bandwidth of the transmitted signal is narrow, known "feedthrough nulling" techniques may be employed to reduce the effects of spillover and such clutter returns. When, however, the bandwidth of the transmitted signal is wide, i.e. when the continuous wave carrier is modulated with a wide band modulation signal to improve range resolution, conventional feedthrough nulling techniques are ineffective because the spectrum of clutter returns may be increased to overlap the spectrum of Doppler frequencies of echo signals from targets desired to be tracked. Obviously, then, in view of the disparity between the levels of clutter returns and echo signals, detection of the latter is most difficult, if not impossible.